Method of coating honeycombed substrates

ABSTRACT

A honeycombed substrate to be coated with a slurry is first heated to a temperature in excess of the boiling point of the slurry liquid medium. The substrate is slowly dipped into the slurry with the axes of the cells extending substantially vertically while maintaining the uppermost ends of the cells above the upper surface of the slurry until the air within the cells has escaped. After the substrate has been removed from the slurry, the substrate is periodically inverted and/or subjected to a series of shakes or jerks while draining under the influence of gravity with the open cell axes extending substantially vertically after each inversion. The substrate is then rotated around a horizontal axis of rotation which extends substantially parallel with the axes of the substrate cells. Forced air is passed through the cells from alternate ends of the substrate until the slurry no longer flows within the cells. The coated substrate is then dried and fired.

United States Patent [191 Dwyer et a1.

[451 Mar. 25, 1975 METHOD OF COATING HONEYCOMBED SUBSTRATES [75]Inventors: Thomas J. Dwyer, Painted Post;

George P. Pesansky, Beaver Dams,

21 Appl. No.1 333,641

[52] US. Cl 117/95, 117/47 H, 117/98, 117/102 R,117/113,117/l19,117/119.8, 117/123 A, 117/123 B, 117/169 A, 117/169 HEATSUBSTRATE FIRE HEAT COATED fi COATED SUBSTRATE SUBSTRATE 3,451,8416/1969 Kesten et a1. 117/98 3,565,830 2/1971 Keith et a1. l17/169 R X3,671,302 6/1972 Nell et al. 117/125 X 3,720,543 3/1973 Bockstie3,790,654 2/1974 Bagley 264/177 Primary ExaminerWilliam D. MartinAssistant Examiner-Shrive P. Beck Attorney, Agent, or Firm-Richard N.Wardell; Norman L. Norris; Clarence R. Patty, Jr.

[57] ABSTRACT A honeycombed substrate to be coated with a slurry isfirst heated to a temperature in excess of the boiling point of theslurry liquid medium. The substrate is slowly dipped into the slurrywith the axes of the cells extending substantially vertically whilemaintaining the uppermost ends of the cells above the upper surface ofthe slurry until the air within the cells has escaped. After thesubstrate has been removed from the slurry, the substrate isperiodically inverted and/0r subjected to a series of shakes or jerkswhile draining under the influence of gravity with the open cell axesextending substantially vertically after each inversion. The substrateis then rotated around a horizontal axis of rotation which extendssubstantially parallel with the axes of the substrate cells. Forced airis passed through the cells from alternate ends of the substrate untilthe slurry no longer flows within the cells. The coated substrate isthen dried and fired.

20 Claims, 8 Drawing Figures PATENTEDHARZSIBYS mQEmmnm SE8 Cm:

METHOD OF COATING HONEYCOMBED SUBSTRATES BACKGROUND OF THE INVENTIONThis invention relates to the coating of thin-walled honeycombedsubstrates.

Thin-walled honeycombed substrates find extensive use as catalystsupports where a honeycombed sub strate is coated with a thin filmcomprising a high surface area, active metal oxide such as gammaalumina. l-leretofore, considerable difficulty has been encountered inobtaining a thin, uniform coating of the honeycombed substrate. Thedifficulties involved may readily be appreciated when one considers thatthe honeycombed substrate is often times characterized by 200 or morecells per square-inch of cross-section where the cells walls are lessthan 0.02 inch thick. It is of course somewhat difficult to obtain auniform coating on the walls of these extremely small cells.

However, the uniform coating of these cell walls is particularlyimportant in many applications including catalyst supports where theobjective is to maximize the overall surface area at which reactions canbe promoted by a catalyst such as platinum. If a coating is sufficientlynon-uniform so as to plug various cells of the substrate, the surfacearea of the metal oxide coating is reduced. It is also desirable toprovide reproducibility in the coating of a particular substrate. Inother words, it is desirable to be able to coat substrate aftersubstrate within a batch with a certain specific percentage by weightcoating and to coat substrates of different batches with the samespecific percentage by weight coating.

In the prior art, various techniques have been utilized in an attempt tocoat substrates. In the catalyst support art, these techniques haveincluded spraying and dipping as disclosed, for example, in U.S. Pat.No. 3,565,830 Keith et al. However, we have found that these techniquespracticed by the prior art have not produced a high degree of uniformityor reproducibility in the coating of thin-walled honeycombed substrates.

SUMMARY OF THE INVENTION It is an overall object of this invention toprovide an improved method of applying coatings to the surface of cellwalls of a thin-walled, honeycombed substrate.

It is a more specific object of this invention to provide a method foruniformly coating substrates of this type.

It is also a specific object of this invention to provide a method ofobtaining reproducible coatings on substrates of this type.

In accordance with one important aspect of this invention, a thin-walledhoneycombed substrate is heated prior to dipping in a slurry comprisinga liquid medium and solids maintained in suspension in the medium. Morespecifically, the substrate is heated to a temperature in excess of theboiling point of the liquid medium so as to negate the capillary actionof the substrate cells on the liquid medium. The substrate is thendipped into the slurry with the axes of the cells within the substrateextending at a substantial angle with respect to the surface of theslurry so as to allow the slurry to rise in these cells as the substrateis dipped into the slurry.

In accordance with another important aspect of the invention, the excessslurry is drained from the sub-- strate prior to drying by supportingthe substrate with the axes of the cells extending substantiallyvertically for a period of time and then inverting and/or shaking orjerking the substrate for a period of time so as to avoid the build-upof slurry deposits within the substrate.

In accordance with another important aspect of the invention, theinitially drained substrate is rotated about an axis extending from oneend of the substrate to the other where the axis of rotation issubstantially parallel with the cell axes and forms an angle of 025 withrespect to a horizontal plane. A gaseous drying medium is then forced inone direction through the cells of the substrate and then another untilthe slurry ceases to flow within the cells of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. l(ah) are illustrations of thevarious steps of a method which forms the preferred embodiment of theinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1(a) illustrates athin-walled substrate 10 of a type which may be coated in accordancewith the method of this invention. The substrate illustrated is anextruded, monolithic, ceramic structure comprising a multiplicity ofopen-ended cells 12 having parallel cell axes where the cells extendfrom one end 14 of the substrate to the other end 16. The method ofmaking this particular ceramic substrate is disclosed in copendingapplication Ser. No. 196,986 filed Nov. 9, 1971, now U.S. Pat. No.3,790,654, which is incorporated herein by reference. Substrates of thistype which are sold by Corning Glass Works under the trademark W-l areoften characterized by a cordierite composition, by 200 or more cellsper square inch of cross-sectional area, by cell walls having athickness of 0005-0100 inch and by a porosity of 15-50%.

In accordance with one very important aspect of the invention, thesubstrate as depicted in FIG. 1(b) is now heated in preparation fordipping in a slurry, which has, been previously mixed by rolling for anhour or more or by other suitable means, where the slurry comprises aliquid medium and solids maintained in suspension in the liquid medium.This heating defeats the capillary action of the cells (which haverelatively small crosssectional area perpendicular to their axes) toallow the solids of the slurry to remain uniformly mixed with the liquidmedium as the slurry passes into successive portions of the cells tothereby obtain a uniform coating. In general, the substrate is heated toa temperature in excess of the boiling point of the liquid medium in theslurry and only slightly in excess of that boiling point so as topreclude rapid evaporation of the liquid medium which can cause abuild-up of heavy deposits on the cell walls during the coating process.In this particular embodiment of the invention, the substrate is beingcoated for use as a catalyst support where the slurry comprises a metaloxide in the form of hydrated alumina (a precursor of high surface area,active gamma alumina) and a liquid medium comprising an aromatic organicsolvent in the form of toluene. The heating of the substrate may be donein an air circulating oven where the temperature is maintained atl20-l30 C. which is l0-20 C. above the toluene boiling point. Ingeneral, the temperature for drying should be maintained 250 C. abovethe liquid medium boiling point where the lower temperatures are usedfor low cell densities and higher temperatures are used for high celldensities.

The substrate is now dipped in the slurry comprising the hydratedalumina and the toluene. In order to assure a thermally stable substratecoating with a large surface area and good adherence between the coatingand the substrate, the slurry also comprises a binder and deflocculantin the form of an organo-silicon compound having a polysiloxane chainstructure, such as a silicone or organopolysiloxane resin, containing aplurality of silanol groups. In a preferred embodiment, the slurryconsists essentially of 45-70% by weight of powdered Alcoa C333 aluminatrihydrate, 728% by weight of Dow Corning 804 silicone resin withsolvent and -24% by weight of Baker reagent grade toluene. Since the DowCorning 804 resin includes toluene as a solvent, the slurry consistsessentially of 4570% by weight of Alcoa C333 alumina trihydrate, 4-18%by weight of the polysiloxane resin and 1835% by weight of toluene asdescribed in copending application Ser. No. 333,642, filed Feb. 20, 1973(assigned to the assignee of this invention), which is incorporatedherein by reference. The slurry is mixed by rolling for about 12 hours.

In order to assure a uniform coating, the substrate 10 is dipped slowlyinto a vessel 18 containing the slurry utilizing a pair of tongs 20 asshown in FIG. 1(a). While the substrate 10 is dipped slowly into theslurry, the axes of the cells 12 are maintained at a substantial anglewith respect to the surface 22 of the slurry, preferably 90 with respectthereto or vertical, so as to allow the slurry to rise slowly in thecells 12 thereby expelling the air trapped in the cells. This assuresthat all cells are filled from the bottom rather than filled as overflowfrom the top. In order to assure expulsion of all air, the upper end 14of the substrate 10 is maintained at or slightly above the upper levelof the slurry surface 22 for a period of 10 seconds as depicted in FIG.1(0).

After all of the air has been expelled, the substrate 10 is submergedbeneath the surface 22 of the slurry as depicted in FIG. 1(d) and heldin the submerged position for a period of approximately 2 minutes. Theslurry may be continuously stirred by a magnetically driven stirrerduring dipping to assist in maintaining the alumina in suspension.

In accordance with another important aspect of the invention shown inFIG. 1(e), the slurry is drained from the substrate 10 by supporting thesubstrate 10 by a screen 24 or other suitable means with the axes of thecells extending substantially vertically. The substrate is then invertedperiodically, e.g., every 20 to 100 seconds, several, e.g., 2 to 6,different times, to avoid the build-up of slurry deposits within thesubstrate cells. Shorter length substrates, e.g., 3 inches in length,generally require lesser periods of draining after each inversion butmore inversions while longer lengths of substrate, e.g., 6 inches inlength, require longer periods of draining after each inversion butfewer inversions. Shakes or jerks can be substituted for part or all ofthe inversions.

In accordance with still another important aspect of the invention shownin FIG. 1(f), the initially drained substrate 10 is rotated about anaxis 26 which extends from one end of the substrate to the other wherethe axis of rotation is substantially parallel with the cell axes andforms an angle of 0-25 with respect to a horizontal plane. This isaccomplished by mounting the substrate 10 in a circular chuck 28 whichis driven by a motor located within a motor housing 30 and which rotatesat a speed of l-l5 R.P.M. with 6 R.P.M. being preferred. This rotationachieves uniformity of coating within a cell in a plane perpendicular tothe cell axes.

As also depicted in FIG. 10), a gaseous'drying medium is forced into thecells from the ends of the substrate. The drying medium may comprise airwhere the air is forced into the cells of the substrate by a 12 inchdiameter household fan 32 located approximately 12-18 inches from theend 14 of the substrate 10. In order to assure uniformity in coating ofthe cell walls along the entire length of the cells, the substrate 10 isremoved from the chuck 28 after a period, e.g., 20 seconds, of forcedair drying. The substrate 10 is then turned 180 so as to place the end14 in the chuck 28. Drying then proceeds for another period, e.g., 3minutes, while the substrate 10 rotates after which the substrate isturned 180 again so as to place the end 16 back in the chuck 28. Thisforcing of the air into the cells from alternate ends of the substratewill assure a uniform coating of the cells along the length thereof.When all traces of wetness are gone, in general about 5 minutes, thesubstrate 10 is removed from the chuck 28. If further drying time isrequired, the substrate can again be turned 180.

In order to expeditiously complete the drying of the slurry coatedsubstrate, it is beneficial to heat the substrate 10 by placing it in afurnace which is maintained at approximately C. as depicted by FIG.1(g). The coated substrate is then allowed to remain in the furnace atthis temperature until thoroughly free of volatile liquid, e.g., forapproximately 2 hours. The temperature of the furnace is then raisedapproximately 100 C. per hour until the 600 C. firing temperature isreached. This temperature calcines the alumina trihydrate to form a highsurface area, active alumina, i.e., gamma alumina, while also convertingthe silicone or polysiloxane resin to a silica which serves to bind thecoating to the ceramic substrate.

By coating a honeycombed substrate with the foregoing method, it hasbeen found that substantial uniformity in the coating from area to areawithin a given sub strate is achieved. In this connection, substratesmeasuring 3 inches in length and 1 inch in diameter were coated with themethod described and subsequently cut into designated pieces. Thesepieces were then crushed to -12 mesh particles and submitted forstandard B.E.T. nitrogen-absorption surface area measurements. Thesurface areas were then compiled by location in the substrate tostatistically determine the uniformity of the coating. The followingtable indicates the surface area measurements for various locationswithin the coated substrate where the first letter represents a selectedpartial cross-sectional area of the sample at any selected axialposition on the sample, the middle digit represents an axial position onthe sample and the sec- 0nd and last letter represents the particularhalf of the selected partial area under consideration.

Table l Alumina Coating Distribution Sample Designation Surface Area,Mlgr Table l-Continued Alumina Coating Distribution Sample DesignationSurface Area, M lgr It will be noted that the surface area measurementsvary from 42-64 square meters/gram which represents a fairly uniformcoating of i22% of the average surface area of approximately SOM /gramwhere the error in surface area measurements was Similar results wereachieved when 6 inch lengths of substrate were coated.

In addition to the uniformity of the coating, it has been found that theforegoing method achieves fairly reproducible results from slurry batchto slurry batch. Different slurry batches containing the same percent byweight of toluene produce coatings to within il0% of the coating weight.Good reproducibility for coated substrates prepared from the same batchis achieved if the liquid medium plus resin is replenished after eachcoating to maintain a relatively constant specific gravity for theslurry.

Finally, it has been found that substrates coated in accordance with theforegoing method are characterized by good coating adherence to thesubstrate. As a test of this adherence, the coated substrates wereplaced in an ultrasonic bath and the percent weight loss of coating wasrecorded as function of time. It was found that W-l substrates coatedutilizing the 2:1 ratio of Alcoa C333 alumina to Dow Corning 804silicone resin and 22% added toluene resulted in a coating which showedless than 1% weight loss after 30 minutes in an ultrasonic bath.

in the foregoing, a specific slurry has been described for use incoating the substrate 10 so as to produce a catalyst support. It will ofcourse be appreciated that other slurries may be utilized. Some examplesof such other slurries are those disclosed in the aforesaid copendingapplication Ser. No. 333,642, filed Feb. 20, 1973, and in US. Pat. No.3,565,830. Also, the alumina may be replaced, in whole or in part, byone or more high surface area oxides of metals from the Groups ll, llland IV having an atomic number of 40 or less. It may also be replaced inwhole or in part of by a precursor of such a metal oxide, whichprecursor becomes the desired oxide upon firing of the coated substrate.Moreover, the method described in the foregoing may be utilized in themanufacture of coated hon- 6 eycombed structures for use other than ascatalyst where the slurry differs considerably from those previouslydescribed.

It will also be understood that the substrate 10 need not be a CorningW-l ceramic substrate but may com- I prise any thin-walled honeycombedstructure having open-ended cells wherein it is desirable to achieve a 5uniform and reproducible coating. For example, the substrate maycomprise a metal or plastic material. Also, the cells may be square andhexagonal in crosssection with 900 or more cells per square inch or asfew as or less cells per square inch of cross-section.

Although a preferred embodiment of the invention has been shown anddescribed and alternative embodiments and modifications have beensuggested, it will be understood that the appended claims are intendedto cover all embodiments and modifications which fall within the truespirit and scope of the invention.

What is claimed is:

l. A method of uniformly coating relatively thin porous walls of aceramic honeycombed substrate forming a multiplicity of elongatedopen-ended cells extending longitudinally from one end of the substrateto the other, said cells having substantially parallel cell axes with acell density in excess of 20 cells per square inch of cross-sectionalarea transverse to the cell axes, said method comprising:

mixing a slurry comprising a liquid medium and solids maintained insubstantially uniform suspension in said liquid medium; heating saidsubstrate to a temperature in excess of the boiling point of said liquidmedium; and

dipping one end of said heated substrate in said slurry whilemaintaining a substantial angle between the axes of said cells and thesurface of said liquid medium as said substrate is lowered in saidslurry so as to substantially preclude said slurry from entering saidcells from said other end of said substrate while the air is escapingfrom said other end as said slurry rises in said cells;

said heating of said slurry substantially negating the capillary actionwithin said cells as said substrate is lowered in said slurry so as toallow the solids of said slurry to remain uniformly suspended in saidliquid medium as said slurry coats the cell walls thereby achieving asubstantially uniform coating of said solids on said cell walls.

2. The method of claim 1 wherein the axes of said cells extendsubstantially vertically with respect to the surface of said slurryduring dipping.

3. The method of claim 1 further comprising:

rotating said dipped substrate around an axis of rotation extending fromsaid one end to said other end of said substrate and substantiallyparallel with the axes of said cells so as to flow said slurry on saidwalls of said cells, said axis of rotation forming an angle of 0-25 withrespect to a horizontal plane.

4. The method of claim 3 further comprising: forcing a gaseous dryingmedium into said cells from alternate ends of said substrate whilerotating said substrate to dry said slurry until the slurry no longerflows on the walls of said cells.

5. The method of claim 4 wherein said gaseous drying medium is alsoforced over the exterior of said substrate.

6. The method of claim 1 further comprising:

draining excess slurry from said substrate by supporting said substratewith said cell axes extending substantially vertically and periodicallyinverting said substrate so as to allow excess slurry to drain from bothends of said substrate.

7. The method of claim 1 wherein said substrate is heated to atemperature of 250C. in excess of the boiling point of said liquidmedium.

8. The method of claim 1 wherein said solids comprise a high surfacearea oxide of a metal.

9. The method of claim 1 wherein said solids comprise a precursor of ahigh surface area oxide of a metal and including the step of firing thedried coating on the substrate so as to convert said precursor to saidoxide.

10. The method of claim 9 wherein said precursor comprises hydratedalumina.

11. The method of claim 10 wherein said slurry further comprises anorgano-silicon compound having a polysiloxane chain structure containinga plurality of silanol groups and said liquid medium comprises anorganic solvent.

12. The method of claim 1 further comprising:

rotating the coated substrate around an axis of rotation extending fromsaid one end to said other end of said substrate so as to flow saidslurry on the walls of said cells, said axis of rotation forming anangle of 025 with respect to a horizontal plane; and

forcing a gaseous drying medium into said cells while rotating saidsubstrate to dry said slurry.

13. The method of claim 12 wherein said gaseous drying medium is forcedinto said cells from alternate ends of said substrate and over theexterior of said substrate while rotating said substrate to dry saidslurry until said slurry no longer flows on said cell walls.

14. The method of claim 12 wherein said substrate is rotated at 1-15revolutions per minute.

15. The method of claim 12 wherein said gaseous drying medium comprisesair.

16. The method of claim 12 further comprising:

draining excess slurry from said substrate after coat- .ing and beforerotating said substrate by supporting said substrate with said cell axesextending substantially vertically and periodically inverting saidsubstrate so as to allow excess slurry to drain from both ends of saidsubstrate.

17. The method of claim 12 wherein said solids comprise at least onesubstance selected from the group consisting of( 1) high surface areaoxides of metals, and (2) precursors of said metal oxides.

18. The method of claim 17 wherein said substance is a said precursorand including the step of firing the dried coating on the substrate soas to convert said precursor to said oxide.

19. The method of claim 18 wherein the precursor comprises hydratedalumina and the metal oxide comprises gamma alumina.

20. The method of claim 19 wherein said slurry further comprises anorgano-silicon compound having a polysiloxane chain structure containinga plurality of silanol groups and said liquid medium of said slurrycomprises an organic solvent.

1. A METHOD OF UNIFORMLY COATING RELATIVELY THIN POROUS WALLS OF ACERAMIC HONEYCOMBED SUBSTRATE FORMING A MULTIPLICITY OF ELONGATEDOPEN-ENDED CELLS EXTENDING LONGITUDINALLY FROM ONE END OF THE SUBSTRATETO THE OTHER, SAID CELLS HAVING SUBSTANTIALLY PARALLEL CELL AXES WITH ACELL DENSITY IN EXCESS OF 20 CELLS OER SQUARE INCH OF CROSS-SECTIONALAREA TRANSVERSE TO THE CELL AXES, SAID METHOD COMPRISING: MIXING ASLURRY COMPRISING A LIQUID MEDIUM AND SOLIDS MAINTAINED IN SUBSTANTIALLYUNIFORM SUSPENSION IN SAID LIQUID MEDIUM; HEATING SAID SUBSTRATE TO ATEMPERATURE IN EXCESS OF THE BOILING POINT OF SAID LIQUID MEDIUM; ANDDIPPING ONE END OF SAID HEATED SUBSTRATE IN SAID SLURRY WHILEMAINTAINING A SUBSTANTIAL ANGLE BETWEEN THE AXES OF SAID CELLS AND THESURFACE OF SAID LIQUID MEDIUM AS SAID SUBSTRATE IS LOWERED IN SAIDSLURRY SO AS TO SUBSTANTIALLY PRECLUDE SAID SLURRY FROM ENTERING SAIDCELLS FROM SAID OTHER END OF SAID SUBSTRATE WHILE THE AIR IS ESCAPINGFROM SAID OTHER END AS SAID SLURRY RISES IN SAID CELLS; SAID HEATING OFSAID SLURRY SUBSTANTIALLY NEGATING THE CAPILLARY ACTION WITHIN SAIDCELLS AS SAID SUBSTRATE IS LOWERED IN SAID SLURRY SO AS TO ALLOW THESOLIDS OF SAID SLURRY TO REMAIN UNIFROMLY SUSPENDED IN SAID LIQUIDMEDIUM AS SAID SLURRY COATS THE CELL WALLS THEREBY ACHIEVING ASUBSTANTIALLY UNIFORM COATING SAID SOLIDS ON SAID CELL WALLS.
 2. Themethod of claim 1 wherein the axes of said cells extend substantiallyvertically with respect to the surface of said slurry during dipping. 3.The method of claim 1 further comprising: rotating said dipped substratearound an axis of rotation extending from said one end to said other endof said substrate and substantially parallel with the axes of said cellsso as to flow said slurry on said walls of said cells, said axis ofrotation forming an angle of 0*-25* with respect to a horizontal plane.4. The method of claim 3 further comprising: forcing a gaseous dryingmedium into said cells from alternate ends of said substrate whilerotating said substrate to dry said slurry until the slurry no longerflows on the walls of said cells.
 5. The method of claim 4 wherein saidgaseous drying medium is also forced over the exterior of saidsubstrate.
 6. The method of claim 1 further comprising: draining excessslurry from said substrate by supporting said substrate with said cellaxes extending substantially vertically and periodically inverting saidsubstrate so as to allow excess slurry to drain from both ends of saidsubstrate.
 7. The method of claim 1 wherein said substrate is heated toa temperature of 2*-50*C. in excess of the boiling point of said liquidmedium.
 8. The method of claim 1 wherein said solids comprise a highsurface area oxide of a metal.
 9. The method of claim 1 wherein saidsolids comprise a precursor of a high surface area oxide of a metal andincluding the step of firing the dried coating on the substrate so as toconvert said precursor to said oxide.
 10. The method of claim 9 whereinsaid precursor comprises hydrated alumina.
 11. The method of claim 10wherein said slurry further comprises an organo-silicon compound havinga polysiloxane chain structure containing a plurality of silanol groupsand said liquid medium comprises an organic solvent.
 12. The method ofclaim 1 further comprising: rotating the coated substrate around an axisof rotation extending from said one end to said other end of saidsubstrate so as to flow said slurry on the walls of said cells, saidaxis of rotation forming an angle of 0*-25* with respect to a horizontalplane; and forcing a gaseous drying medium into said cells whilerotating said substrate to dry said slurry.
 13. The method of claim 12wherein said gaseous drying medium is forced into said cells fromalternate ends of said substrate and over the exterior of said substratewhile rotating said substrate to dry said slurry until said slurry nolonger flows on said cell walls.
 14. The method of claim 12 wherein saidsubstrate is rotated at 1-15 revolutions per minute.
 15. The method ofclaim 12 wherein said gaseous drying medium comprises air.
 16. Themethod of claim 12 further comprising: draining excess slurry from saidsubstrate after coating and before rotating said substrate by supportingsaid substrate with said cell axes extending substantially verticallyand periodically inverting said substrate so as to allow excess slurryto drain from both ends of said substrate.
 17. The method of claim 12wherein said solids comprise at least one substance selected from thegroup consisting of (1) high surface area oxides of metals, and (2)precursors of said metal oxides.
 18. The method of claim 17 wherein saidsubstance is a said precursor and including the step of firing the driedcoating on the substrate so as to convert said precursor to said oxide.19. The method of claim 18 wherein the precursor comprises hydratedalumina and the metal oxide comprises gamma alumina.
 20. The method ofclaim 19 wherein said slurry further comprises an organo-siliconcompound having a polysiloxane chain structure containing a plurality ofsilanol groups and said liquid medium of said slurry comprises anorganic solvent.